In general, an electrothermal transducer (heater) and its drive circuit of a recording head mounted on a recording apparatus compliant with an inkjet method are formed on the same substrate by using a semiconductor process technology as indicated in U.S. Pat. No. 6,290,334 for instance. There is a proposal of a configuration of the recording head having a digital circuit for detecting a state of a semiconductor substrate such as a substrate temperature for instance formed on the same substrate in addition to the drive circuit and also having an ink supply port around the center of the substrate and the heaters at opposed positions across the port.
FIG. 1 is a diagram schematically showing circuit blocks and the ink supply ports of this kind of inkjet recording head substrate (head substrate). FIG. 1 shows the semiconductor substrate of a head substrate 114 on which six ink supply ports 111 are formed. For convenience sake, FIG. 1 only shows the circuit blocks corresponding to the ink supply ports 111 on the left side and omits showing circuit blocks (115) corresponding to the other five ink supply ports 111. As shown in FIG. 1, heaters 110 are placed like an array at opposed positions across the ink supply ports 111. The circuit blocks (drive circuits 113) for selectively driving the heaters 110 are placed correspondingly to the heaters 110. Pads 102 for supplying power and signals to the heaters 110 and drive circuits 113 are placed at ends of the semiconductor substrate 114.
FIG. 2 is a diagram showing one of the supply port circuit blocks 115 shown in FIG. 1 in further detail together with a flow of an electrical signal. As shown in FIG. 2 the circuit blocks (drive circuits 113 of FIG. 1) are symmetrically placed centering on the ink supply port 111. The circuit blocks include a drive circuit array 109, a drive selection circuit array 108, a device driving signal circuit 104, a block selection circuit 105 and bus lines 106, 107 described later. The heater arrays 110 are provided across the ink supply port 111 and comprise multiple heaters. The drive circuit array 109 has a switching device for passing a current to each heater of the heater array 110. The drive selection circuit array 108 controls the drive circuit. The device driving signal circuit (also referred to as a time division selection circuit) 104 and block selection circuit 105 generate the signals transmitted to the drive selection circuit array 108. An input circuit 103 processes the signals inputted from the pads 102.
Hereunder, a description will be given as to functions of each of the circuit blocks and flows of the signals of one circuit block group which is symmetrical centering on the ink supply port 111.
A head substrate 101 is a silicon substrate on which the circuit blocks and the heaters for heating ink are formed by using an LSI process. The power supply voltage and signals inputted from the pads 102 for inputting and outputting image data are transmitted to the device driving signal circuit 104 and block selection circuit 105 via the input circuit 103. The signals appropriately processed by the device driving signal circuit 104 and block selection circuit 105 are led to a heater row direction by the bus lines 106 and 107 consisting of multiple lines.
The signals from the bus lines 106 and 107 are connected to drive selection circuits which are components of the drive selection circuit array 108 respectively. On and off of drive selection circuits are decided by the signals from the bus lines 106 and 107. In the case of performing a discharge operation of the ink, the signal for turning on a desired drive selection circuit is applied to the bus lines 106 and 107, and the signal is outputted from the drive selection circuit to turn on a corresponding drive circuit in the drive circuit array 109. The turned-on drive circuit passes a current to the corresponding heater in the heater array 110. The heater is heated by this current, and bubbling and discharge operations of the ink are performed.
FIG. 3 schematically shows more detailed circuit configuration and flows of signals of the drive circuits 113 of FIG. 1 (drive circuit array 109, drive selection circuit array 108, device driving signal circuit 104, block selection circuit 105 and bus lines 106, 107 of FIG. 2). The shown example indicates a state in which the drive circuit array 109 and drive selection circuit array 108 are configured by eight heater driving blocks 206. The signals including the image data and time division data applied to the pads 102 are inputted to the block selection circuit (mainly configured by a shift register) 105 and device driving signal circuit (mainly configured by a decoder) 104 configuring an internal circuit via the input circuit 103. In the example shown in FIG. 3, the inputted time division data is converted to a time division selection signal (also referred to as a device driving signal) by the device driving signal circuit 104. The time division selection signal is supplied to each of the heater driving blocks 1 to 8 (206). The block selection circuit 105 generates a block selection signal for selecting the heater driving blocks 1 to 8 based on an image data signal synchronized with a synchronizing signal (clock) used to input the image data. The heater driving block selected by the block selection signal drives the heater according to the time division selection signal. To be more specific, the heater to be driven is decided by an AND of the block selection signal and the time division selection signal.
FIG. 4 shows a detailed configuration of the heater driving block 206. The heater driving block 206 includes heater driving MOS transistors 209, level conversion circuits 205 and heater selection circuits 204 placed correspondingly to heaters 210 placed like an array. Here, the heater driving MOS transistor 209 performs a function of a switch for turning on and off energization of the heater 210. The drive selection circuit array 108 of FIG. 2 corresponds to the heater selection circuits 204 and level conversion circuits 205. The drive circuit array 109 corresponds to the heater driving MOS transistor 209. A block selection signal 202 from the block selection circuit 105 and a time division selection signal 203 from the device driving signal circuit 104 are inputted to an AND gate of the heater selection circuit 204. Therefore, in the case where both the signals 202 and 203 become active, an output of that AND gate becomes active. An output signal of the AND gate has its voltage amplitude level-converted by the level conversion circuits 205 to the power supply voltage (second power supply voltage) higher than a driving voltage (first power supply voltage) from the input circuit 103 to the heater selection circuits 204. The level-converted signal is applied to a gate of the heater driving MOS transistor 209. The heater 210 connected to the heater driving MOS transistor 209 having the signal applied to its gate has a current passed thereto and driven. The second power supply voltage is level-converted by the level conversion circuits 205 for the purposes of increasing the voltage applied to the gate of the heater driving MOS transistor 209 and thereby reducing its on-resistance and passing the current to the heater with high efficiency.
FIG. 5 is a circuit diagram showing the drive selection circuit and drive circuit corresponding to one arbitrary heater 210 in the heater array 110 extracted from the above-mentioned drive selection circuit array 108 and drive circuit array 109. FIG. 5 describes a detailed circuit configuration of the heater selection circuits 204 and level conversion circuits 205 shown in FIG. 4.
The signals are taken into the drive selection circuit from the bus lines 106, 107 leading the output signals from the device driving signal circuit 104 and block selection circuit 105 shown in FIG. 2. Reference characters 208a to 208l denote circuit elements configuring the drive selection circuits (heater selection circuit 204 and level conversion circuit 205). An input terminal of an NAND gate 208a (heater selection circuit 204) is connected to the bus lines 106 and 107 and a corresponding signal is inputted from each bus line. An inverter 208b outputs the signal having inverted the output signal from the NAND gate 208a, and an inverter 208c further inverts the inversion signal. The MOS transistors 208d to 208i configure a level converter for converting the voltage amplitude of the signals. The MOS transistors 208j, 208k configure the inverter for buffering the output signals of the level converter. There is also a resistance 208l provided to increase an output impedance when the output of the inverter formed by the MOS transistors 208j, 208k shifts from a low level (hereafter, indicated as Lo) to a high level (hereafter, indicated as Hi).
A MOS transistor 209 forms the drive circuit for exerting on and off control of a heater current. Heating by the heater 210 for ink foaming is controlled by on and off of the heater current by the MOS transistor 209.
Operations of the circuits shown in FIG. 5 will be described. The output signals from the device driving signal circuit 104 and block selection circuit 105 of FIGS. 2 and 3 are inputted to the NAND gate 208a by the bus lines 106 and 107. Here, the output of the NAND gate 208a becomes Lo only when both the inputs to the NAND gate 208a become Hi. Hereunder, the operation in the case where a Lo signal is outputted from the NAND gate 208a will be described. The Lo signal outputted from the NAND gate 208a is inverted by the inverter 208b to become Hi. Furthermore, the Hi signal as the output of the inverter 208b is inputted to the inverter 208c and is inverted again to be outputted as the Lo signal. The voltage amplitudes of the bus lines 106 and 107, NAND gate 208a, inverters 208b and 208c are VDD (first power supply voltage) of which potential has the same amplitude as the signals inputted from outside.
The output signals from the inverters 208b and 208c are in putted to the level converter including the MOS transistors 208d to 208i respectively. Here, the potential of Lo (0 V) which is the same as the output signal of the NAND gate 208a is applied to the gates of the MOS transistors 208d and 208e, and the potential of Hi (VDD) which is the inversion signal of the output of the NAND gate is applied to the gates of 208g and 208h. 
The MOS transistor 208g having VDD applied to its gate is an NMOS transistor, and so it becomes on-state. For that reason, a drain terminal of the NMOS transistor 208g is connected to a GND potential at low impedance. The drain terminal of the NMOS transistor 208g is connected to the gate of a PMOS transistor 208f. For that reason, the gate of the PMOS transistor 208f is connected to the GND potential at low impedance, and the PMOS transistor 208f becomes on-state. The PMOS transistor 208e series-connected to the PMOS transistor 208f is on-state because 0 V is applied to the gate. The NMOS transistor 208d further series-connected is off-state because 0 V is applied to the gate. As the PMOS transistors 208f, 208e are on and the NMOS transistors 208d is off, the potential at the drain of the PMOS transistors 208e is VDDM. Therefore, the potential of a node having the drains of the PMOS transistor 208e and NMOS transistor 208d and the gate of the PMOS transistor 208i connected thereto becomes VDDM (second power supply voltage) which is a power supply potential of the level conversion circuits. For that reason, the PMOS transistor 208i becomes off-state. To be more specific, the PMOS transistor 208i becomes off and the NMOS transistor 208g becomes on. For that reason, the drain terminals of the PMOS transistor 208g and NMOS transistor 208i are connected, and the potential of the node connected to the gate of the PMOS transistor 208f is fixed at 0 V. The potential of the node becomes the output signal of the level converter, and is inputted to the gates of the inverter consisting of the NMOS transistor 208j and PMOS transistor 208k. 
Thus, if 0 V is applied to the gates of the transistors of the inverter consisting of the NMOS transistor 208j and PMOS transistors 208k, the PMOS transistors 208k becomes on and the NMOS transistor 208j becomes off. Consequently, the inverter outputs the VDDM potential so that VDDM is applied to the gate of the NMOS transistor 209 which is the drive circuit for exerting on and off control of the heater. The NMOS transistor 209 having VDDM applied to its gate becomes on-state and passes the heater current from a heater power supply potential VH via the heater 210. The heater having the current passed through generates the heat necessary for the ink foaming and discharge.
Thus, the heater current passes when both the signals connected from the bus lines 106 and 107 to the NAND gate 208a become Hi.
Here, the resistance 208l is placed to curb a precipitous rising edge of the heater current. To be more specific, in the case where a gate potential of the NMOS transistor 209 for exerting on and off control of the heater current transits instantaneously from 0 V to the VDDM potential for the sake of turning on the heater current, the heater current also passes instantaneously. There are the cases where this change of the current becomes noise of the power supply and triggers a malfunction. The resistance 208l is inserted between the PMOS transistor 208k and the NMOS transistor 209 in order to prevent the malfunction. As the precipitous rising edge of the gate potential of the NMOS transistor 209 is curbed by a lagged effect of the on-resistance of the PMOS transistor 208k, series resistance of the resistance 208l and gate capacity of the NMOS transistor 209, an instantaneous flow of the heater current is curbed to prevent the malfunction.
FIG. 6 shows a portion equivalent to the level conversion circuit 205 extracted from the circuit shown in FIG. 5 (the resistance 208l is omitted). As shown in FIG. 6, the level conversion circuits 205 is divided into a circuit portion 205a for operating at the first power supply voltage (VDD) and a circuit portion 205b for operating at the second power supply voltage (VDDM). A heater selection signal 221 which is the output from the heater selection circuit 204 is inputted to the inverter 208b (configured by a PMOS transistor 230 and a NMOS transistor 231) for operating at the first power supply voltage. The inverter 208b generates a signal of an inversion logic of the heater selection signal 221, and applies it to the gates of the NMOS transistor 208g and PMOS transistor 208h operating at the second power supply voltage. The inversion signal of the inverter 208b is inputted to the inverter 208c to be inverted again. The output signals of the inverter 208c are applied to the gates of the NMOS transistor 208d and PMOS transistor 208e operating at the second power supply voltage. The circuit portion 205b outputs the signal converted to an amplitude of the second power supply voltage (VDDM) according to these signals.
As for the inkjet recording heads in general, the number of nozzles is increased and density thereof is furthered for the purposes of speeding up recording and/or improving a grade of recording. In the case of a thermal inkjet printer for discharging the ink by generating heat with the heater as mentioned above, however, it is necessary to use a high power supply voltage in order to have the energy required for the ink foaming and the discharge in conjunction therewith generated by the heater. Therefore, as to a drive control circuit of the heater, it is necessary for the component devices such as the transistors to secure a withstand voltage against the high power supply voltage. In general, size of each component device increases for the sake of securing the withstand voltage of the device so that a high-density (small-layout-pitch) circuit layout on the substrate becomes difficult.
For instance, the conventional circuit form as shown in FIG. 5 takes an AND of the signal transmitted from the device driving signal circuit 104 via the bus line 106 and the block selection signal transmitted from the block selection circuit 105 via the bus line 107. The voltage amplitude is increased as to the signals after taking the AND.
Such a configuration requires the circuit block for operating at the first power supply voltage (VDD) which is the voltage amplitude of the input signal and the circuit block for operating at a higher second power supply voltage (VDDM) to be applied to the gate of the MOS transistor for controlling the heater current. To be more specific, the head substrate must have the configuration for each of the heaters wherein it is controlled and driven at two kinds of power supply voltages, that is, the first and second power supply voltages, and a signal amplitude of the first power supply voltage is converted to that of the second power supply voltage by the level conversion circuit. For this reason, the level conversion circuit described in FIG. 6 is provided to each heater drive MOS transistor. However, such a level conversion circuit is configured by a large number of transistors, and so the required area of chips becomes large in the case of the configuration having the level conversion circuit for each individual heater.
As each of the heaters requires the level conversion circuit, it is necessary to place a large number of devices of a high withstand voltage. For this reason, a high-density (small-layout-pitch) device layout on the substrate becomes difficult. To be more specific, the layout pitch cannot be sufficiently reduced because of existence of the large number of devices of a high withstand voltage, resulting in an increase in a chip size.
As each of the heaters requires the level conversion circuit, it is necessary to place a large number of devices of a high withstand voltage. For this reason, a high-density (small-layout-pitch) device layout on the substrate becomes difficult. To be more specific, the layout pitch cannot be sufficiently reduced because of existence of the large number of devices of a high withstand voltage, resulting in an increase in a chip size.
The devices of a high withstand voltage of FIG. 5 are the level converter and inverter (circuit portion 205b) connected to VDDM which is a midpoint potential and the transistor (209) for driving the heater connected to VH.
Therefore, when considering a layout configuration of a recording head substrate of the above-mentioned configuration, the level conversion circuit added to each segment leads to an increase in length of each segment and an increase in the chip size and becomes a factor of increased costs. To be more specific the above-mentioned layout expands the chips in the direction orthogonal to the heater array so that the chips increase remarkably. An increase in the number of circuit elements leads to reduction in yield and further complexity of the circuit configuration, which become the factors of further increased costs.